1. Field of the Invention
The present invention relates to a control device for an internal combustion engine (engine) and a method of controlling the internal combustion engine in a vehicle on which an engine automatic stop/start control system (idle-stop control system) for controlling stop and restart of the internal combustion engine is mounted.
2. Description of the Related Art
In recent years, the number of vehicles, on which an engine automatic stop/start control system, so-called idle-stop control system is mounted, is increasing for the purposes of improvement of fuel efficiency and a reduction in gas emission. A general idle-stop control system stops fuel injection (performs fuel-cut) to automatically stop an internal combustion engine (engine) when a driver stops a vehicle. Thereafter, when the driver performs an operation to start the vehicle, for example, a brake-pedal release operation or an accelerator-pedal depressing operation, the idle-stop control system automatically energizes a starter or a motor also serving as a starter to crank the engine to restart the vehicle.
In the idle-stop control system described above, a restart request is sometimes generated immediately after the generation of an automatic stop request while an engine rotation speed is being reduced by the fuel-cut. In this case, if the starter is energized to crank the engine to restart the engine after the rotation of the engine is completely stopped, time from the generation of the automatic stop request to the completion of the restart becomes long. As a result, a delay (slowness) in restart becomes undesirably noticeable to the driver.
Therefore, the following engine restart has been proposed. When the engine rotation speed is within the range in which the restart of the engine is enabled only by the fuel injection in the case where the restart request is generated while the engine rotation speed is being reduced by the fuel-cut in the idle-stop control, the engine is restarted (self-recovered) only by the fuel injection without using the starter.
On the other hand, when the engine rotation speed at the time of generation of the restart request is below the range of rotation speed in which the engine can be self-recovered, the engine is required to be restarted by energizing the starter even before the rotation of the engine is stopped. If an air amount in cylinders is small at this time, a fluctuation in engine rotation speed just before the rotation is stopped becomes small to reduce vibrations of the engine (vehicle body). Therefore, a feeling of discomfort experienced by the driver can be reduced. On the other hand, when the air amount in the cylinders is large, the fluctuation in engine rotation speed becomes large to give a feeling of discomfort due to the vibrations to the driver.
In the case where the starter is a push-out type starter in which a pinion gear is pushed out to be meshed with a ring gear, when a difference between a rotation speed of the ring gear and that of the pinion gear is small at the time of meshing between the ring gear and the pinion gear, the meshing can be achieved within a short period of time. Therefore, an unpleasant noise generated at the time of meshing is unlikely to be noticed by the driver. On the other hand, when the difference in rotation speed is large, a long time is required to achieve the meshing. As a result, the unpleasant noise generated at the time of meshing is likely to be noticed by the driver. Moreover, when the range of variation in the engine rotation speed is large, the difference in rotation speed greatly changes to hardly achieve the meshing. As a result, the unpleasant noise generated at the time of meshing is likely to be noticed by the driver.
On the other hand, during the fuel-cut, a throttle-valve opening degree is controlled so that the throttle valve is in a fully-closed position. There is a response delay from the generation of the restart request until the throttle-valve opening degree is increased from that in the fully-closed position to a target opening degree at the time of restart to increase the air amount in the cylinders to a requested air amount for the restart. The response delay of an intake system is significantly larger than that of a fuel injection system. Therefore, even if the fuel injection is restarted immediately in response to the generation of the restart request, the increase in air amount in the cylinders is delayed due to the response delay of the intake system, resulting in a small combustion torque of the engine at the time of restart.
Therefore, in the range of rotation speed in which the engine can be self-recovered, a frequency (probability) of failing in restart becomes higher. Further, even in the restart of the engine with the starter, the time from the restart to the recovery of the engine rotation speed becomes long. In order to prevent the aforementioned disadvantages, it is necessary to ensure a given air amount in the cylinders, that is, to set an intake-pipe pressure to a given value or larger before the generation of the restart request.
Specifically, in order to achieve the conflicting objects, that is, to ensure restartability while the fluctuation in engine rotation speed is reduced, it is necessary to control the air amount in the cylinders to fall within a given range, that is, to control the intake-pipe pressure to fall within a given range in preparation for the restart request, which may be generated at any time by a request of the driver after the generation of the automatic stop request.
Therefore, the following automatic stop/start control device for the internal combustion engine has been proposed. The automatic stop/start control device sets the control amount of the intake system, for example, the throttle-valve opening degree to the opening side (on the side on which the air amount increases) of the throttle-valve opening degree at the time of generation of the automatic stop request. The set throttle-valve opening degree enables a reduction of the fluctuation in engine rotation speed immediately before the stop of the engine rotation to reduce the vibrations of the engine while a necessary intake air amount is ensured to ensure restartability (for example, see Japanese Patent Application Laid-open No. 2010-242621). In the control device described above, the driving of the throttle-valve opening degree to the closing side along with a reduction in engine rotation speed has also been proposed. In this manner, the vibrations of the engine due to the fluctuation in engine rotation speed are further reduced.
However, the related art has the following problems.
In a process of the reduction of the engine rotation speed in response to the generation of the automatic stop request, an intake-pipe pressure is a pressure which achieves a balance between the air amount sucked into the cylinders through the throttle valve and the air amount exhausted from the cylinders to an exhaust pipe. In the aforementioned process, the intake-pipe pressure gradually increases as the engine rotation speed decreases. Finally, the intake-pipe pressure becomes equal to an atmospheric pressure.
The air amount passing through the throttle valve is determined by the throttle-valve opening degree and a difference between a pressure at upstream of the throttle valve and a pressure at downstream thereof. Therefore, as the throttle-valve opening degree becomes smaller, the air amount to be introduced into the intake pipe is reduced. Therefore, the degree of increase in intake-pipe pressure due to the reduction in engine rotation speed becomes smaller. Moreover, the throttle-valve opening degree at the time of generation of the automatic stop request is generally set to the opening degree that ensures the air amount which allows an idle state of the engine to be maintained (air amount for Idle Speed Control (ISC)).
In the automatic stop/start control device described in Japanese Patent Application Laid-open No. 2010-242621 cited above, both in the case where the throttle-valve opening degree is set constant and in the case where the throttle-valve opening degree is driven to the closing side with the reduction in engine rotation speed, the throttle-valve opening degree is not set on the closing side of the throttle-valve opening degree at the time of generation of the automatic stop request. Therefore, the air amount passing through the throttle valve is ensured to be equal to or larger than the air amount for ISC. As a result, the degree of increase in intake-pipe pressure due to the reduction in engine rotation speed becomes relatively large.
Accordingly, although the intake-pipe pressure is desired to be controlled to fall within a given range in order to achieve the conflicting objects described above, the time which allows the given range to be ensured becomes relatively short. As a result, the restart request is generated when the intake-pipe pressure is lower than the range in which the intake-pipe pressure is desired to fall under the control, or the intake-pipe pressure just before the stop of the rotation of the engine becomes higher than the range in which the intake-pipe pressure is desired to fall under the control. As a result, there is a problem in that the fluctuation in engine rotation speed becomes large.
Further, in the automatic stop/start control device described in Japanese Patent Application Laid-open No. 2010-242621 cited above, in the case where the throttle-valve opening degree is set constant, the throttle-valve opening degree cannot be set greatly larger than the throttle-valve opening degree at the time of generation of the automatic stop request because the throttle-valve opening degree is required to be set so as to achieve the conflicting objects described above. Therefore, there is another problem in that along time is required for the intake-pipe pressure to reach the range in which the intake-pipe pressure is desired to fall under the control. Moreover, if the throttle-valve opening degree is first set large and then is driven to the closing side with the reduction in engine rotation speed, a change in engine rotation speed which is set as a condition for driving the throttle-valve opening degree to the closing side does not actually occur.